Evaporation

Evaporation is a process of diffusion of the particles of a substance, either liquid or solid, into the surrounding medium. If from the solid state the process is called sublimation. If the transition from liquid to vapour, or from vapour to liquid, be perfectly free, which can only occur when the pressure of the vapour is sufficient to overcome the external pressure, the process is called ebullition, or boiling. [Ebullition.] At any pressure a certain amount of evaporation may occur. Thus, even ice may evaporate, though it is obviously far below its boiling-point; and in course of time may entirely disappear. The amount of evaporation at a temperature below boiling-point depends chiefly on that temperature, but other local conditions, such as pressure, may also hold. Heat is required to effect the change from the liquid to the gaseous state, and inasmuch as this heat disappears as such, it is termed the latent heat of evaporation. If t is fhe temperature of free evolution of water-vapour Regnault gives the amount of latent heat as 606.5 + .305 t, the temperature and the heat unit being in degrees Centigrade. This production of cold, for as such the subtraction of heat may be regarded, is found practically useful. Liquid carbon dioxide may be in part frozen by the evaporation of part, a process that is applied in various other cases. [Heat.]